The high toxicity of nitrogen oxides and their role in the formation of acid rain and tropospheric ozone have resulted in the imposition of strict standards limiting the discharges of these chemical species. To meet these standards, it is generally necessary to remove at least part of these oxides present in the exhaust gases from stationary or mobile combustion sources.
Denitration or selective catalytic reduction (SCR) technology is commonly applied to combustion-derived flue gases for removal of nitrogen oxides. The denitration reaction comprises the reaction of nitrogen oxide species in the gases, such as nitrogen oxide (NO) or nitrogen dioxide (NO2), with a nitrogen containing reductant, such as ammonia or urea, resulting in the production of diatomic nitrogen (N2) and water.
In addition to nitrogen oxides, sulfur dioxide (SO2) is a chemical species often present in combustion-flue gases that causes great environmental concern. Sulfur dioxide that is present in fossil fuel combustion flue-gases is partly oxidized to sulfur trioxide (SO3) which reacts with water to form sulfuric acid. The formation of sulfuric acid from the oxidation of sulfur dioxide in combustion flue-gases can increase corrosion problems in downstream equipment, can increase power costs associated with air pre-heaters due to the increased temperature required to keep the acid-containing flue-gas above its dew point, and can cause increased opacity in the stack gases emitted to the atmosphere.
Catalyst systems for the removal of nitrogen oxides can increase the amount of sulfur dioxide oxidation since the catalytic material utilized in selective catalytic reduction can additionally effectuate the oxidation of sulfur dioxide. As a result, the reduction in the nitrogen oxide content of a combustion flue-gas can have an undesirable side-effect of increasing SO3 formation in the combustion flue-gas.
Combustion flue-gases containing nitrogen oxides and a significant sulfur dioxide content are commonly produced from the combustion of coal. Coal-fired combustion flue-gases contain high amounts of particulate matter, especially in the form of ash. This particulate matter has the ability to clog the cells of a monolithic structural catalyst body resulting in a reduced catalytic performance and efficiency. Individual ash particles alone can plug catalyst cells or ash particles can aggregate to produce a plug. Moreover, smaller particulate matter can plug catalytic pores located within inner partition walls of the catalyst body.
It would be desirable to provide a monolithic structural catalyst body comprising a high open frontal area (OFA) catalyst structure and composition that demonstrates a superior utilization of catalyst activity for the selective reduction of nitrogen oxides while minimizing the oxidation of sulfur dioxide wherein the catalyst structure resists plugging by particulate matter.